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Lac Operon: Gene Regulation in E.Coli
Overview: The lac operon is an operon present within E.coli cells (and other bacteria) that is responsible for the creation of proteins that can be used to breakdown and utilize lactose as an alternative energy source to glucose. This operon is fully functioning when there is no glucose present, but plenty of lactose. It works by transcribing the genes needed to make the proteins that will breakdown lactose and use it as an alternative energy source to glucose. The lac operon system is made up of a LacI regulatory gene (codes for LacI repressor), an activator-binding site (where the activator binds), a promoter (encourages RNA polymerase binding), an operator (where repressor binds), and three structural genes called LacZ (cleaves lactose), LacY (transports lactose into cells), and LacA (unknown function). Each of these elements contributes to the operon’s function as a whole and plays an important role in regulating when this operon is transcribed and when it is not. Key Players of Regulation: Catabolite Activator Protein (CAP)- This protein, when bound by cAMP, will bind to the activator-binding site of the operon and enhance transcription. Cyclic Adenosine Monophosphate (cAMP)- this signaling molecule is present when glucose, the preferred energy source, is scare within the cell. It will bind to CAP and activate it, which in turn activates the transcription of the lac operon. LacI Repressor- this repressor protein, when 'NOT '''bound by allolactose, will bind to the operator of the lac operon and inhibit its transcription by blocking the RNA polymerase. Allolactose- this lactose isomer molecule will be present within the cell when lactose is present and will bind the LacI repressor causing it to dissociate from the operator and allow transcription of the lac operon to occur. Key Stimuli/Mechanisms of Expression: ]There are two key stimuli that effect the Lac gene’s regulation: glucose and lactose. Glucose, when low within the cell, will cause an increase in cAMP, which will then bind the CAP and cause it to activate the transcription of the Lac operon. When glucose is high within the cell, there will not be a lot of cAMP present and therefore the CAP will not be bound by cAMP and in turn not activate transcription of the Lac operon. Lactose, when low within the cell, will cause a decrease in allolactose, which means it will not bind the LacI repressor and therefore the repressor will remain bound to the operator and halt transcription of the Lac operon. Lastly, when there is a lot of lactose within the cell, allolactose will be highly present and bind to the LacI repressor causing it to dissociate from the operator and allow transcription of the Lac operon to occur. In summary, both glucose and lactose are the key stimuli in regards to regulating the amount of Lac operon gene expression. Watch this video for an additional overview of the lac operon gene expression process: http://www.youtube.com/watch?v=2sMFswbOgKk Significance of Regulation: The lac operon and its ability to be regulated is very important within ''E.Coli cells when it comes to energy production, utilization, and conservation. E.Coli ''cells prefer to use glucose as their energy source because it is able to provide more energy than lactose when broken down. If glucose is present, the lac operon will be negatively regulated in order to save energy. When glucose becomes scarce within the ''E.Coli ''cell, the lac operon is positively regulated in order to take advantage of the lactose present. Although lactose doesn’t produce as much energy as glucose when broken down, it’s better than not creating any energy at all. The fact that the lac operon has regulation provided by an activator and a repressor allows for fine tuning of the operon. Collectively, the activator and repressor work together to regulate the gene expression of the lac operon with help from their associated activator molecules (cAMP for the CAP and allolactose for the LacI repressor). Resources: # ''The Lac Repressor by M. Lewis, C.R. Biologies (2005) PMID: 15950160 # Structural explanation for allolactose (lac operon inducer) synthesis by lacZ β-galactosidase and the evolutionary relationship between allolactose synthesis and the lac repressor by Wheatley RW, Lo S, Jancewicz LJ, Dugdale ML, & Huber RE (2013) PMID: 23486479 # Lecture 16: Transcription Regulation by Prof. Laura Almstead (UVM) # http://www.Wikipedia.org # The Lac Regulatory Gene taken from http://www.phschool.com Images/Video: # # http://diaryofanalevelstudent.wordpress.com/2013/02/23/the-lac-operon/ # http://en.wikipedia.org/wiki/Lac_operon # http://bio1903.nicerweb.com/Locked/media/ch18/lac_CAP.html # http://peda.gov.in/eng/energy_conservation.html # http://www.youtube.com/watch?v=2sMFswbOgKk